Small Bar Specimens Research Articles

大型溶接構造物の微小き裂進展と疲労寿命

To develop a method for evaluating the fatigue life of large structural components by using the data obtained with bar specimens 10mm in diameter, a bending fatigue test was performed on a large butt-welded joint plate of a 50mm thick hot-rolled SUS304 stainless steel made by hot-wire TIG welding. This test confirmed the evaluation method proposed and showed the followings.(1) The peak strain range in the welded metal becomes 1.8 times as large as the average strain range of welds because of the metallurgical discontinuity in the welded metal and the different deformation characteristics of the base and weld metals. As a result, fatigue cracks initiate where the peak strain occurs.(2) The most part of fatigue life of large structural components is crack propagation process, so that it is reasonable to evaluate fatigue life based on crack length.(3) Scale factors between the large structural component and the small bar specimens can be calculated using the fatigue failure probability of the small bar specimens, by taking the ratio of two test region areas into consideration.(4) If the crack initiation life corresponding to a surface crack length less than 2mm is used, the life evaluation method based either on the base metal data or on the welded joint data obtained using small bar specimens is reasonable.

Strain and Fatigue Strength of Rack Tooth Fillet for Leg Fixing Device of Jackup Rigs

The rack teeth of a leg fixing device, which is indispensable for jackup rig to operate in harsh environment, take cyclic load caused by waves and winds. Accordingly, it is very important to know the fatigue strength of rack teeth.In this paper, fatigue tests of rack tooth models and small bar specimens whose material is HT 80 are performed, and the analysis of the rack tooth models by finite element method is performed as elasto-plastic contact problem with friction.Comparing the calculated results with experimental ones, following results are obained.(1) The effect of frictional force on the strain at rack tooth fillet is not negligible.(2) Manson-Coffin type equation proposed by Iida is useful to predict the fatigue crack initiation life for HT 80.

A fracture criterion for nuclear graphite

Measurements of tensile and flexural strength of a nuclear graphite using small bar specimens are reported. A theoretical model of failure is described in which critical defects are assumed to be associated with cleaved large coke particles. It is further assumed that these subcritical cracks can only extend by the cleavage of small particles which surround them. The size of the critical flaws is defined on the basis of linear elastic fracture mechanics. The model predicts the observed variation in flexural strength in three- and four-point bending as the knife edge spacings are varied. It slightly overestimates the observed ratio of tensile-to-bend strength, and this is thought to be due to some of the simplifying assumptions made in the models formulation. The potential of the model for use in engineering applications is discussed.

An Investigation of Low Cycle Fatigue Life of Wide Plate with a Notch

In the previous paper, a pulsating tension test was carried out using wide plate specimens of mild steel with a central notch of stress concentration factor Kt to be 5 and 10. By simulating the cyclic behaviors of the strain at the tip of a notch to hour-glass shaped small specimens, the low-cycle fatigue crack initiation life of a plate specimen was compared with that of a small bar specimen. The results showed good correlation between the wide plate and the small bar specimen, but the life of the plate specimen with a notch of 5 in Kt was slightly longer than that of small bar specimen, while that for notch of 10 in Kt was slightly shorter. This fact shows that the crack initiation life depends on not only the cyclic behavior of strain but also the configulation of a notch.In this paper, a same test was carried out using wide plate specimens and small bar specimens of mild steel and 50 kg/mm2 high tensile steel, and the crack initiation life was investigated more precisely based on the present and previous results. It was revealed that dominant factors affecting the crack initiation at the tip of notch in the pulsating tension test are maximum strain at the first loading, strain amplitude, increasing mean strain and restraint of deformation at the tip of a notch. Since the influence of the restraint of deformation at the tip of notch can not be realized in the strain controlled test simulating the cyclic behavior of strain at the tip of a notch, the larger Kt of a notch is, the shorter the crack initiation life of the plate specimen is than that of simulating test on the small bar specimen. By comparing with the results of completely reversed strain-cycling test of small bar specimens, it was found that mean strain and its increase show supper posed effect on the crack initiation life when the life of the plate specimen is shorter than about 700 cycles. As for the effect of the increasing mean strain, an effect of the increasing mean strain was found to be appeared only when the increasing rate of mean strain is larger than about 0.1%/cycle, from the results of comparing test of linearly increasing mean strain with that of constant mean strain.In this paper, two cumulative damage rules in low-cycle fatigue are proposed taking into consideration of four factors affecting the crack initiation. One is based on the cyclic behavior of strain, and the other on the cyclic behavior of notch opening displacement. These two rules are able to estimate the fatigue life of the plate specimen within a small extent of deviation.

Initiation and Propagation of a Crack and Cyclic Behavior of Strain in Low-Cycle Fatigue

The cyclic behaviors of strain in mild steel plate specimens with a central notch were investigated in static tensile test and zero-to-tension cyclic loading test. In the cyclic test a cumulative damage law was applied to the crack initiation life by using the strain behavior at the tip of the notch, and satisfactory results were obtained. The strain controlled fatigue test simulating the strain behavior at the tip of notch was carried out by using small bar specimens of the same material, and a good correlation was found between the crack initiation life of the plate specimen and that of the bar specimen. By comparing the fatigue test of a linearly increasing mean strain and that of a constant mean strain, the crack initiation life in the increasing mean strain test was shorter than that in the constant mean strain test.The fatigue crack propagation in the plate specimen was observed under the condition of a constant, stepwise increased or stepwise decreased net section stress range. The crack propagation rate was correlated to the maximum strain at the tip of a crack or the stress intensity factor range. The characteristics of the crack propagation and the strain behavior after the stepwise decrease of net section stress range were very different from the other condition of net section stress range.